1. Field of the Invention
The present invention relates to a pin mirror cutter for machining or "working" a crankshaft of a reciprocating internal combustion engine, and more particularly, to a mounting mechanism for a pin mirror cutter for attaching and detaching a cutter body of the pin mirror cutter to and from a cutter mounting portion provided in a working machine.
2. Description of the Related Art
Pin mirror cutters, such as that shown in FIG. 3, have generally been known for use as a tool for working a crankshaft of a reciprocating internal combustion engine. As shown in the figure, pin mirror cutter 1 is detachably mounted onto the inner periphery of a ring-shaped cutter body 2 by means of bolts 5 with a plurality of chips 3 having cutting edges 4 projected from the inner peripheral surface of the cutter body 2.
The pin mirror cutter 1 is mounted to cutter mounting portion 7 provided in the working machine with its axis aligned so as to be substantially coincident with a shaft (not shown), as shown in FIG. 4. In addition, a crankshaft (not shown), which is to be machined, passes through an inner hollow section to be stretched between chucks 9 and 10 provided on the working machine 6.
In operation, the pin mirror cutter 1 is rotated about its axis in a fixed direction (in the direction of the arrow A of FIG. 3) by the shaft. In addition, while being rotated about its axis around the fixed crankshaft, the pin mirror cutter is moved in the axial direction, thereby working the crankshaft into a predetermined shape by the cutting edges 4 of the pin mirror cutter 1. In any method for working the crankshaft using such a pin mirror cutter 1, the alignment, or "coaxiality" between the axis of the pin mirror cutter 1 and a rotation axis of the shaft of the working machine 6 has a significant impact on the cutting or working accuracy.
For this reason, as shown in FIGS. 3 and 5, an annular step portion 11, coaxial with the shaft, is formed on the inner periphery of the cutter mounting portion 7, and an annular flange 12 is formed on the outer periphery of the cutter body 2 so as to be fitted with each other. Furthermore, concave fitting portions 13 are formed in a wall 11a facing the radial center of the step portion 11, and four key members 14 (one of them is omitted from FIG. 3) each projecting from the wall 11a, are fitted to the concave fitting portions 13. The key members 14 are typically formed in a rectangular parallelepiped and spaced at intervals of 90.degree. around the inner periphery of the cutter mounting portion 7. Typically, the key members 14 are fitted to the respective concave fitting portions 13 which are precisely positioned so that center lines m in the width direction of the respective key members 14 intersect at a radial center P, and are fixed by bolts 15.
In addition, key grooves 17, each having the same width as the key members 14 and a rectangular cross section, are formed on the outer periphery of the cutter body 2 in an arrangement similar to that of the concave fitting portions 13. The key members 14 are fitted to the key grooves 17, so as to prevent radial movement and ensure coaxiality of the cutter body 2 with respect to the step portion 11 of the cutter mounting portion 7.
In mounting the cutter body 2 to the cutter mounting portion 7, the flange 12 of the cutter body 2 is fitted to the step portion 11 of the cutter mounting portion 7 and the key members 14 are fitted into the key grooves 17, whereby the cutter body 2 is mounted to the cutter mounting portion 7 in such a manner that the axis of the cutter body 2 coincides with the axis of the step portion 11. In addition, substantially semicircular recesses 18a and 18b constituting circular recesses 18 are formed in the flange 12 and the step portion 11, respectively, and clamps 19 having substantially semicircular shape are rotatably fitted to the circular recess 18. Once the clamps 19 are rotated in the recess 18a of the flange 12, whereby the cutter body 2 is firmly fixed to the cutter mounting portion 7, the mounting of the pin mirror cutter 1 to the working machine 6 is completed.
In a pin mirror cutter of this type, if the cutter body 2 is mounted to the cutter mounting portion 7 without precise alignment between the axis of the cutter body 2 with the axis of the cutter mounting portion 7, the workpiece cannot be worked into a truly round shape. Therefore, it is necessary to precisely center the cutter body 2 each time such is installed on the mounting portion 7.
Thus, according to the above mounting mechanism, the step portion 11 formed on the inner periphery of the cutter mounting portion 7 and the flange 12 formed on the outer periphery of the cutter body 2 are fitted to each other. Key members 14 are then fitted to the concave fitting portions 13 formed on the wall 11a of the step portion 11 and to the key grooves 17 formed in the flange 12 of the cutter body 2, thereby preventing radial movement and ensuring coaxiality of the cutter body 2 with respect to the step portion 11 of the cutter mounting portion 7.
Such a mounting mechanism, however, inherently suffers from reduced rigidity because the thicknesses of the step portion 11 and the flange 12 are about half the thickness of the cutter body 2. In addition, the depth of the circular recesses 18 further requires a reduction in the thickness of the flange 12, so that a sufficient thickness of the clamps 19 fitted to the circular recesses 18 cannot be provided, thereby compromising the rigidity of the clamps 19.
For the above reasons, it has been found that when the cutter body 2 having a lesser thickness is used in accordance with the size of the workpiece or when the cutting load increases, such as during high-speed and rapid-feed cutting, the cutter body 2 tends to chatter, thereby causing breakage of the chips 3 during cutting, and compromising working accuracy.
In particular, when the cutter body having a lesser thickness is used, the thickness of the key members 14 in contact with the grooves and fitting portions for transmitting a rotating torque of the cutter mounting portion 7 also decreases, so that the driving surfaces through which the torque is transmitted, i.e., contact areas among the key members 14, the concave fitting portions 13 and the key grooves 17, decrease and the torque is not efficiently transmitted from the cutter mounting portion 7 to the cutter body 2.
Furthermore, in the above mounting mechanism, a fixed clearance is provided between the flange 12 of the cutter body 2 and the step portion 11 of the cutter mounting portion 7, so as to allow easy mounting of the cutter body 2. However, such a clearance makes it more difficult to precisely mount the cutter body 2 to the cutter mounting portion 7 so that the axes thereof coincide with each other, and it has been found that the centering operation requires a great deal of labor.